Optical systems are presently being employed in the communication of voice and video information as well as in the high speed transmission of data. Optical communication systems are desired because of the wide bandwidth available for the information signal channels.
Although this wide bandwidth is available, many of the existing optical fiber systems use only a single channel per optical fiber. Typically, this channel is transmitted at a wavelength of 1310 nm in one direction from a transmitting end to a receiving end and requires a second optical fiber to achieve bidirectional communication; however, recent increase in telecommunications traffic has resulted in a need for further fiber resources. One way this need was met, was to install additional optical fiber cables. Another was to increase the number of channels carried by same fibers.
Recently, technologies that can add additional channels to existing optical fiber cables already in the ground, have gained acceptance. These technologies seek to provide more than one channel on a single existing optical fiber and are therefore aimed at enhancing the efficiency of the existing fiber optic cable network. These technologies include wavelength division multiplexing (WDM) and bidirectional transmission.
When a number of wavelengths are multiplexed and transmitted on a single optical fiber, customarily, these channels must later be demultiplexed into separate channels or wavelengths of light. For example, it may be cost effective to transmit signals of wavelength .lambda.1, .lambda.2, .lambda.3, .lambda.4, .lambda.5, and .lambda.6 (.lambda. denoting a wavelength, lambda) along a single optical fiber, however, demultiplexing means are required to separate the light into six separate channels. Of course, it is desired to perform this demultiplexing at a minimum cost and with as little signal loss as possible.
Various types of optical filters have been contemplated and used to separate light of differing wavelengths. Unfortunately, coupling and other losses associated with many of these arrangements have led to devices that are less than satisfactory. For example, dichoric filters are widely used as WDM devices; however, the reduction of channel spacing as well as the passband is limited by the current interference filter technology.
As of late, in-fiber Bragg gratings have become more prevalent in the field of fiber optics. An optical system utilizing Bragg gratings in combination with an optical circulator is shown in U.S. Pat. No. 5,283,686 issued Feb. 1, 1994 in the name of David Huber and assigned to General Instrument Corporation, Jerrold Communications, Hatboro, Pa. However, there are certain problems associated with the use of Bragg gratings; for example, the coupling losses in utilizing Bragg gratings and circulators alone as a means of multiplexing/demultiplexing in high density applications may in some instances be prohibitive. Furthermore, low yield, and cost are further disadvantages.
Fibre Bragg gratings have been used to compensate for dispersion present in optical signals. One such circuit is described in a paper entitled Dispersion Compensation Over Distances in Excess of 500 km for 10-Gb/s Systems Using Chirped Fiber Gratings, by W. H. Loh, et al in IEEE Photonics Technology Letters, Vol. 8, No, 7 July 1996 incorporated herein by reference.
Bragg gratings have been know to be used in add-drop optical circuits, where it is desired to drop a particular channel from a group of multiplexed channels, and to subsequently add-in a channel having the same wavelength as the dropped channel. However, one known problem with circuits of this type is that unwanted group delay occurs when "strong" Bragg gratings are used. Such "strong" gratings are reflective gratings having a steep wavelength response designed to reflect an entire predetermined band or channel with high isolation from adjacent transmitted channels, thereby providing minimal crosstalk.
Heretofore, group delay or dispersion has been in part remedied by the use of dispersion compensation fibre, or other means of dispersion compensation, however the group delay relating to "strong" Bragg gratings is complex non-linear phenomenon, and is not remedied only by use of dispersion compensated optical fibre.
It is an object of this invention to provide a drop circuit that is capable of dropping a predetermined channels from a multi-channel signal with less loss group delay within the dropped channel band than prior art devices that use Bragg gratings and provide high isolation.
It is an object of the invention to provide a convenient relatively inexpensive circuit for providing an add-drop function with minimized group delay.
Accordingly, in a preferred embodiment the present invention provides an optical drop circuit for dropping a channel n comprising a band of wavelengths of light centered about a wavelength .lambda.n from a signal including channel n and a plurality of other channels comprising bands of different wavelengths of light, and for providing group delay compensation to the dropped channel n comprising:
an input port for introducing the signal; PA1 a first Bragg grating having a strong reflective response for separating substantially all wavelengths of light in the band corresponding to channel n from the wavelengths of light corresponding to the plurality of other channels in the signal; PA1 a pass-through port for transmitting the wavelengths of light corresponding to the plurality of other channels separated by the first Bragg grating; PA1 a second Bragg grating having a lower reflective response than the first Bragg grating for separating the wavelengths of light corresponding to channel n from the different wavelengths of light in the plurality of other channels, said second Bragg grating having a period length and refractive index difference to lessen group delay for the band centered about the wavelength .lambda.n introduced by the first Bragg grating for providing group delay compensation for wavelengths of light corresponding to channel n; PA1 isolation means between the first and the second filter means; and, PA1 an output port for transmitting the wavelengths of light corresponding to channel n separated and provided with group delay compensation by the second Bragg grating. PA1 an input port for inserting the signal into the circuit; PA1 a first Bragg grating having a strong reflective response for reflecting substantially all the channel n wavelengths and transmitting a remaining portion of the signal; PA1 first output means for transmitting the remaining portion of the signal; PA1 means for introducing the new channel n into the remaining portion of the signal; PA1 a second Bragg grating having a reflective response lower than the first Bragg grating for reflecting the channel n wavelengths and transmitting other wavelengths of light, and for providing group delay compensation for the at least one channel n; PA1 isolation means between the first filter means and the second filter means; and, PA1 at least one output port for transmitting the channel n. PA1 a first optical circulator having at least four circulator ports including an input port, and output port and a plurality of intermediate sequential ports for circulating optical signals from the input port to a next sequential port in a circulating direction; and PA1 a first Bragg grating filter having a strong reflective response coupled to one of said sequential ports for reflecting a selected portion of said optical signals in a predetermined wavelength band, and for transmitting another portion of said optical signals at other wavelengths outside the predetermined wavelength band; and PA1 a second Bragg grating filter having a reflective response lower that the first Bragg grating coupled to another of the sequential ports for transmitting or reflecting the selected portion of the optical signals in the predetermined wavelength band and for providing group delay compensation to lessen group delay in the selected portion of the optical signal. PA1 an input port for launching a composite multiplexed optical signal comprising signals corresponding to the plurality of channels and a signal corresponding to the nth channel; PA1 an output port optically coupled with the input port to receive signals corresponding to the plurality of channels in a direction from the input port to the output port, light being prevented from passing in a reverse direction from the output port to the input port; PA1 a drop port for receiving light corresponding to the nth channel launched into the input port that has not propagated to the output port; PA1 a Bragg grating filter disposed between the input port and the output port having a response for separating substantially all wavelengths of light in the band corresponding to channel n from the wavelengths of light corresponding to the plurality of other channels in the signal; PA1 a Bragg grating filter between the output port and the drop port having a response for separating the wavelengths of light corresponding to channel n from the different wavelengths of light in the plurality of other channels, and for providing group delay compensation for wavelengths of light corresponding to channel n; and, PA1 isolation means between the two Bragg grating filters for preventing light from passing from the output port to the input port and for preventing unwanted etalon effects from occurring between the two Bragg grating filters.
In a further preferred embodiment the present invention provides an add/drop optical circuit for dropping a channel n comprising a band of wavelengths of light centered about a wavelength .lambda.n from a signal including a plurality of channels comprising bands of different wavelengths of light, for providing group delay compensation to the dropped channel n, and for introducing a new channel n replacing the dropped channel n comprising:
A still further preferred embodiment provides a multiple wavelength optical filtering device for optical signal transmission, said device comprising:
A preferred embodiment may also comprise an optical drop circuit for dropping a channel n comprising a band of wavelengths of light centered about a wavelength .lambda.n from a signal including channel n and a plurality of other channels comprising bands of different wavelengths of light, and for providing group delay compensation to the dropped channel n comprising:
Advantageously this invention provides separates the function of providing high rejection of the wavelengths associated with a dropped signal channel from a through path and the function of providing low adjacent channel cross talk and substantially constant dispersion by use of a multiple gratings.
Advantageously, the separation of these functions into plural gratings for which the combined response gives the desired filtering function in transmission, reflection and dispersion. The gratings are designed to compensate for each other particularly with regard to dispersion. A wide range of grating combinations can be considered to cover a range of complex functions.